a) Field of the Invention
The present invention relates to a hydrodynamic bearing apparatus which is structured such that a shaft member and a bearing member rotatably support each other by hydrodynamic pressure generated in a lubricant.
b) Description of the Related Art
In recent years, various hydrodynamic bearing apparatus using the hydrodynamics of lubricants, such as oil and the like, are studied and provided to respond to high-speed rotations in various apparatus such as motors and the like. These hydrodynamic bearing apparatus are such that the hydrodynamic surface on the side of a shaft member is placed across from the hydrodynamic surface on the side of a bearing member which is fit around or to the end of the shaft member via a predetermined space, at least one of the facing hydrodynamic surfaces comprises a hydrodynamic generating groove and the pressure of a lubricant between the facing surfaces of the shaft member and bearing member is increased by a pumping action of the hydrodynamic generating groove during rotation such that both members are supported in relation to each other by the hydrodynamic pressure of the lubricant.
Hydrodynamic bearing apparatus, as described above, comprise lubricants such as oil and the like (simply "lubricants" hereafter) in the bearing section; there are two types of support structures for the lubricants:
1) Partial lubricant filling structure
This is the simplest structure for a hydrodynamic bearing in which a lubricant fills only the bearing section and in which an air layer is formed between bearings. For example, in an apparatus described in Tokkai S58-50318, two hydrodynamic bearing sections are separately formed in the axial direction while a lubricant filling the inside of the two hydrodynamic bearing sections is separated by an air layer. In such a hydrodynamic bearing apparatus having a separated lubricant, the lubricant inside each hydrodynamic bearing section is maintained by surface tension while the air layer between the hydrodynamic bearing sections is open to the atmosphere via a continuing clearance hole formed on the shaft member.
2) Lubricant circulating structure
This is a structure in which the lubricant also fills the space between the bearings and both ends of the bearings are connected by a circulating hole. Therein, the lubricant is circulated so that internal pressure differences (differential pressures) generated during rotation are nullified. According to this structure, it is possible to maintain a sufficient amount of the lubricant to prolong the life of the bearings; also, it is advantageous to prevent the lubricant from leaking externally since the internal pressure differences (differential pressures) of the lubricant are always nullified by the circulating hole.
Each of the above mentioned conventional hydrodynamic bearing structures, however, has problems. In the case of the partial lubricant filling structure 1), it is difficult to control the injected amount of the lubricant, and the absolute amount of the lubricant is small due to the small space in the bearing section, causing the following problems. First, initial start up causes friction in the bearing; friction powder or sludge mixed into the lubricant increases the viscosity of the lubricant such that the bearing characteristics are deteriorated. Also, the amount of the lubricant circulated during rotation is low such that the temperature of the lubricant tends to increase, resulting in deterioration caused by heat. As a result, the life of the bearing is shortened. If the filling space for the lubricant is increased to increase the overall amount of the lubricant, the lubricant tends to leak. Furthermore, since the volume of the air layer expands/contracts due to the changes in the pressure and the temperature of the air layer between the bearings such that moving and leaks of the lubricant are the results. In order to prevent this condition, it is necessary to provide a hole continuing to the outside and the like.
On the other hand, in the case of the lubricant circulating structure 2), as explained above, the structure becomes complicated due to the circulating hole, causing poor productivity and high manufacturing costs.